This study is designed to address questions regarding the possible association between the intake of various dietary components and cancer of the breast and large bowel, as well as other disease outcomes. This is to be done by taking advantage of an existing prospective study of risk factors for breast cancer in 121,000 women who complete a mail questionnaire on a biennial basis. This investigation will require the development of a short dietary questionnaire, administration of this questionnaire to the entire cohort, and subsequent follow-up on a biennial basis to gather data on the development of specific cancers of interest. The questionnaire will be designed to provide relative scores for individuals on: intake of animal fat, vegetable fat, cholesterol, protein, fiber, Vitamin A, and "trans" fatty acids. Data on potential confounding factors, including physical activity, for risk of breast cancer and large bowel cancer is being collected and will be controlled by multiple logistic analyses. To validate the questionnaire, 200 subjects will complete detailed 7-day food use diaries four times over a l year period. Blood specimen from 400 women at the extremes of consumption histories will be assessed for lipids, carotene, and vitamin A levels. These specimens will be preserved for possible subsequent endocrinologic determinations. Outcome measures will include both fatal and non-fatal cancer incidence rates. Reported cases of cancer will be documented by obtaining copies of hospital records and causes of death will be obtained from death certificates as well as hospital records. Over the course of 4 years of follow-up, sufficient cases of breast and large bowel cancers are expected to occur in the cohort to permit a better than 90% chance of detecting a 1.5-fold risk for breast cancer and a 2-fold risk of large bowel cancer.